Define an equivalent duty before comparing equipment
Specify water flow and quality, process loading, the dissolved oxygen target and periods of peak demand. A side stream can add oxygen without replacing all reactor mixing. If an option requires additional mixers or aeration, their consumption belongs within the comparison.
Do not compare a rapid DO increase in clean water with maintaining a biological process in industrial wastewater. EPA guidance distinguishes factors affecting transfer, while USGS provides saturation calculations for temperature, pressure and salinity. Final DO concentration is also not a direct measure of the oxygen mass transferred over an entire trial.
Convert gas volume into oxygen mass
A flow of m³/h measured in a pressurised line is not the same as m³/h referred to normal conditions. A quotation should state reference temperature, absolute pressure and dry or wet basis, as well as the flowmeter calibration for the gas. Nm³/h without that information leaves a significant ambiguity.
For an original calculation, assume dry gas at 0 °C and 101.325 kPa absolute, behaving ideally. Using the O₂ molar mass published by NIST, approximately 32 g/mol, calculated oxygen density is 1.43 kg/m³. Oxygen supply is referenced gas flow × oxygen mole fraction × 1.43.
On that basis, 20 m³/h contains about 6.0 kg O₂/h with an assumed air composition of 20.9 % O₂, or 27.2 kg O₂/h with assumed gas purity of 95 %. These compositions are example assumptions. Equal volume does not mean equal supply; a different reference requires recalculation.
Separate oxygen supplied from oxygen transferred
Gas entering the equipment may leave without dissolving. Calculate utilisation as net oxygen mass transferred to the liquid divided by O₂ mass supplied over the same interval. Measuring it may require gas and liquid balances, including escape and biological uptake. A single DO reading is insufficient.
Higher oxygen content changes transfer conditions but does not guarantee a particular efficiency. Test the combined gas supply, generator, injector, pump and water. Conventional aeration results or clean-water tests do not automatically establish the performance of a nanobubble side stream.
Include complete power and supply costs
Count the recirculation pump, compressor, cooling, drying and controls. If oxygen is produced on site, add the generator and its auxiliaries. For purchased oxygen, state cost per delivered kg and supply losses; do not describe external manufacturing energy as electricity measured within the facility.
This energy example is independent of the earlier volume calculation and assumes different, unspecified gas flow rates. A hypothetical air option consumes 20 kW and transfers 10 kg O₂/h, giving 2.0 kWh/kg. An oxygen option consumes 8 kW in the circuit and 15 kW in generation while also transferring 10 kg/h, giving 2.3 kWh/kg overall. These are assumptions, not product performance figures. Comparing only 8 with 20 kW would change the conclusion.
Record consumption at partial load, startup and peak demand, as well as average operation. Compare annual costs using the same operating hours and required availability.
What an acceptance trial should establish
Agree water flow, gas analysis, DO locations, temperature, wastewater loading and electricity meters before starting. Record auxiliary equipment remaining in operation and the mixing required. Separate measured data, conversions and estimates in the report.
Confirm that the complete assembly is designed for the chosen gas and service pressure. Select the option that meets the process consistently within a transparent economic boundary. Nominal bubble size alone does not establish universal savings in gas or electricity.
Common questions
Can Nm³/h of air be compared with Nm³/h of oxygen?
Only after confirming identical reference conditions and converting the O₂ content to mass. Purity and water content affect the result.
Does an oxygen unit with lower power always cost less?
No. Generation, purchased gas, auxiliaries or additional mixing may be missing. Compare the complete service.